Aquatitan

Berserk. Whenever the aquatitan starts its turn with 40 hit points or fewer, roll a d6. On a 6, the aquatitan goes berserk. On each of its turns while berserk, the golem attacks the nearest creature it can see. If no creature is near enough to move to and attack, the aquatitan attacks an object, with preference for an object smaller than itself. Once the aquatitan goes berserk, it continues to do so until it is destroyed or regains all its hit points.

The aquatitan's creator, if within 60 feet of the berserk aquatitan, can try to calm it by speaking firmly and persuasively. The golem must be able to hear its creator, who must take an action to make a DC 15 Charisma (Persuasion) check. If the check succeeds, the golem ceases being berserk. If it takes damage while still at 40 hit points or fewer, the golem might go berserk again.

Aversion of Ligntning. If the aquatitan takes lightning damage, it has a disadvantage on attack rolls and ability checks until the end of its next turn.

Immutable Form. The aquatitan is immune to any spell or effect that would alter its form.

Amphibious. The aquatitan can breathe air and water.

Watery Advantage. While submerged in liquid, the aquatitan has advantage on attack rolls.

Magic Resistance. The aquatitan has advantage on saving throws against spells and other magical effects.

Magic Weapons. The aquatitan's weapon attacks are magical.

Mucous Cloud. While underwater, the aquatitan is surrounded by transformative mucus. A creature that touches the aquatitan or that hits it with a melee attack while within 5 feet of it must make a DC 14 Constitution saving throw. On a failure, the creature is diseased for 1d4 hours. The diseased creature can breathe only underwater.

Multiattack. The aquatitan makes two slam attacks.

Slam. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage.

Drown. The aquatitan may choose to grapple and drag another creature to the nearest body of water, when doing so the aquatitan makes their first grapple Athletics (Strength) check with disadvantage, if they win the Grapple contest the targeted creature is Grappled and Drowned (or Strangled). If successful, the suffocating creature may attempt to break the Grapple on its turn, ending the drowning.